1. Field of the Invention
This invention pertains to electric energy measuring circuits and more specifically to electronic watthour metering circuits employing digital techniques for computing the product of voltage and current components.
2. Description of the Prior Art
The electromechanical rotating disc-type of watthour meter continues to be used almost exclusively in electrical metering applications. However, alternate metering methods are being sought in order to facilitate functions such as time-of-day metering and load control.
The electrical energy supplied to a circuit is the product of measured power and the period of time during which that power level is measured. AC power can be determined by multiplying the peak voltage and peak current together with the power factor. This method requires circuitry for measuring the peak values of the voltage and current components. Additional circuitry is needed to determine the phase angle between the voltage and current components and for generating a cosine function to produce the power factor.
Much of this circuitry was eliminated in U.S. Pat. No. 4,056,774 for "An Electronic Watthour Meter Circuit" issued Nov. 1, 1977 and assigned to the assignee of this invention, which described a circuit that sampled both the current component and the voltage component at a time when the voltage component was at its peak value. Bandpass filters are used to smooth out the input wave forms to facilitate this peak detection. This type of sampling provided a peak value for the voltage component and a value for the current component which was equal to the peak value of the current component multiplied by the cosine of the phase angle between the voltage and current components thus eliminating the need for phase angle detection and generation of the cosine function. While this circuit finds uses in various applications it may be desirable to sample at a faster or different sampling rate. It may also be desirable not to filter the input wave forms. These features and others are incorporated in the present invention.